A civil liberties campaign group today began a court fight with Government intelligence services after saying it believed that its private communications had been "interfered with" in breach of human rights legislation.

Lawyers started to outline details of Liberty's claims against the Government Communications Headquarters (GCHQ), the Secret Intelligence Service - MI6, and the Security Service - MI5, at a special tribunal in London.

The public hearing before the Investigatory Powers Tribunal, which investigates complaints about the conduct of the security and intelligence services and often sits behind closed doors, is expected to end later this week.

Two Green Party politicians have already launched similar claims.

Detail of claims by Brighton Pavilion MP Caroline Lucas and Baroness Jones, a member of the London Assembly, was given to the Investigatory Powers Tribunal at a public preliminary hearing earlier this month.

The two women have complained that a doctrine - the Wilson Doctrine - which enables MPs to communicate privately with constituents and members of the public has been ''undermined'' by ''modern mass surveillance'' practices.

A document prepared by Government lawyers and shown to tribunal judges at that hearing on July 1 said ministers could "neither confirm nor deny the existence of the alleged 'Tempora' operation".

Government lawyers have produced a glossary of terms contained in documentation for the hearing starting today - to help judges analyse argument.

One acronym in the glossary was "NCDC" - short for "neither confirm nor deny".

Liberty says there is a "reasonable likelihood" that intelligence services have "interfered with" its private communications in breach of rights to private life and freedom of expression.

Officials want the tribunal to declare that GCHQ, MI6 and MI5 acted unlawfully.

Liberty says its claim is based on intelligence services' use of two programmes - PRISM and Tempora. Officials say PRISM is a programme used by the United States National Security Agency (NSA) to obtain data and Tempora a programme apparently used by GCHQ to intercept communications through fibre optic cables entering and leaving the UK.

Matthew Ryder QC, for Liberty, told the tribunal today that legal action had been launched in the wake of revelations by former NSA analyst Edward Snowden.

Lawyers representing ministers are contesting the claim.

James Eadie QC, who heads a Government legal team, told judges in a written submission: "There is a clear need for the intelligence services to be able to share intelligence - on a proportionate basis - with foreign intelligence partners."

He said such work was appropriately governed by statute.

Mr Justice Burton began proceedings by asking for more seats to be provided so that members of the public and journalists did not have to stand.